Bailet Olivier, Fenouille Nina, Abbe Patricia, Robert Guillaume, Rocchi Stéphane, Gonthier Nadège, Denoyelle Christophe, Ticchioni Michel, Ortonne Jean-Paul, Ballotti Robert, Deckert Marcel, Tartare-Deckert Sophie
Institut National de la Santé et de la Recherche Médicale U895, Team 1, Biology and Pathologies of Melanocytes, Nice, France.
Cancer Res. 2009 Apr 1;69(7):2748-56. doi: 10.1158/0008-5472.CAN-08-2690. Epub 2009 Mar 17.
Loss of tumor-suppressive pathways that control cellular senescence is a crucial step in malignant transformation. Spleen tyrosine kinase (Syk) is a cytoplasmic tyrosine kinase that has been recently implicated in tumor suppression of melanoma, a deadly skin cancer derived from pigment-producing melanocytes. However, the mechanism by which Syk suppresses melanoma growth remains unclear. Here, we report that reexpression of Syk in melanoma cells induces a p53-dependent expression of the cyclin-dependent kinase (cdk) inhibitor p21 and a senescence program. We first observed that Syk expression is lost in a subset of melanoma cell lines, primarily by DNA methylation-mediated gene silencing and restored after treatment with the demethylating agent 5-aza-2-deoxycytidine. We analyzed the significance of epigenetic inactivation of Syk and found that reintroduction of Syk in melanoma cells dramatically reduces clonogenic survival and three-dimensional tumor spheroid growth and invasion. Remarkably, melanoma cells reexpressing Syk display hallmarks of senescent cells, including reduction of proliferative activity and DNA synthesis, large and flattened morphology, senescence-associated beta-galactosidase activity, and heterochromatic foci. This phenotype is accompanied by hypophosphorylated retinoblastoma protein (Rb) and accumulation of p21, which depends on functional p53. Our results highlight a new role for Syk tyrosine kinase in regulating cellular senescence and identify Syk-mediated senescence as a novel tumor suppressor pathway the inactivation of which may contribute to melanoma tumorigenicity.
控制细胞衰老的肿瘤抑制途径的丧失是恶性转化的关键步骤。脾酪氨酸激酶（Syk）是一种细胞质酪氨酸激酶，最近被认为与黑色素瘤的肿瘤抑制有关，黑色素瘤是一种源自产生色素的黑素细胞的致命皮肤癌。然而，Syk抑制黑色素瘤生长的机制仍不清楚。在这里，我们报告黑色素瘤细胞中Syk的重新表达诱导细胞周期蛋白依赖性激酶（cdk）抑制剂p21的p53依赖性表达和衰老程序。我们首先观察到Syk表达在一部分黑色素瘤细胞系中丧失，主要是通过DNA甲基化介导的基因沉默，并在用去甲基化剂5-氮杂-2'-脱氧胞苷处理后恢复。我们分析了Syk表观遗传失活的意义，发现黑色素瘤细胞中重新引入Syk可显著降低克隆形成存活率以及三维肿瘤球体的生长和侵袭。值得注意的是，重新表达Syk的黑色素瘤细胞表现出衰老细胞的特征，包括增殖活性和DNA合成的降低、大而扁平的形态、衰老相关的β-半乳糖苷酶活性以及异染色质灶。这种表型伴随着视网膜母细胞瘤蛋白（Rb）的低磷酸化和p21的积累，这依赖于功能性p53。我们的结果突出了Syk酪氨酸激酶在调节细胞衰老中的新作用，并确定Syk介导的衰老为一种新的肿瘤抑制途径，其失活可能有助于黑色素瘤的致瘤性。
